1. Field of Invention
This invention relates to a control method and device for an electrical discharge machine capable of employing multiple discharge-servo curves, and especially to a numerical control method used by the electrical discharge machine to choose multiple servo curves by calling a servo curve instruction during the machining process. This application can be used for any electrical discharge machine, including a drilling electrical discharge machine, and a wire-cut electrical discharge machine.
2. Description of The Prior Art
An Electrical Discharge Machine (EDM) utilizes the sparking generated between the electrode and workpiece to melt as well as evaporate material, under a locally instant high temperature condition, to achieve the cutting result, which is a non-traditional machining technology. Even though EDM has already been adopted to cut a more complex and higher precision product by industries worldwide, the precision control technique of EDM is still not good enough and needs to be improved
The gap-voltage control-loop of the electrical discharge machine determines the suitable cutting speed by real-time measurement of the movement and calculation of the deviation of the gap-voltage. FIG. 1 is a functional block diagram of a known gap-voltage control loop of the electrical discharge machine, which is arranged to calculate the gap-voltage difference between the real measurement and the operator inputs, and then to output the cutting speed after the outcomes are processed through the servo curve controller and the deviation controller. Furthermore, the positioning control-loop guides the electrical discharge machine to work on the cutting according to a position command integrated by the integrator. Finally, the positioning controlloop issues a real gap dimension for calculating the gap deviation by subtracting the real gap dimension from an original operator-input gap value, calculates the gap-voltage according to the gap deviation using the gap-voltage converter, and then subtracts that gap-voltage from the reference operator-input gap-voltage. The next gap-voltage control cycle is then carried out in an iterative manner, the cutting speed again corresponding to the gap-voltage deviation, which is calculated by the internally used gap-voltage deviation and cutting speed control curve of the servo curve controller. The combined gap-voltage deviation and cutting speed control curve are generally referred-to as a discharge-servo curve. As shown in the FIG. 2, the discharge-servo curve is built on the relationship of the gap-voltage deviation and the cutting speed, and contains a nonlinear discharge-servo curve 1a and a linear discharge-servo curve 1b. 
The servo curve controller of the electrical discharge machine chooses a discharge-servo curve by the control software device of a known electrical discharge machine software structure, as shown in FIG. 3. A flowchart of a known discharge-servo curve control method is shown in FIG. 4. Before initiating the program, the discharge-servo curve is set via the CNC control panel and the discharge-servo curve data is recorded within the servo curve controller, and then a cutting process is carried-out according to the discharge-servo curve data which is recorded within the servo curve controller as soon as the program is started.
The required cutting speed differs during the workpiece cutting process, as a result of different machining environments resulting from factors such as a different material of electrode, workpiece, cutting solution, and coarse or fine process. If the known discharge-servo curve control method is adopted, the executing program has to be stopped in order to exchange discharge-servo curve data, and returned to the step of setting the discharge-servo curve on the CNC control panel for swapping the record within the servo curve controller and the discharge-servo curve data Therefore, it is really not convenient to the case of a single workpiece with servo curves. How to allow an electrical discharge machine arbitrarily to choose the discharge-servo curve during the real cutting process is a significant problem.